Consider a thin, 50 mm X 50 mm fuel cell similar to that of Example 1.5, with air in parallel flow over its surfaces. Very small-diameter wires are stretched across both sides of the fuel cell at a distance x = x_c from the leading edge in order to trip the flow into turbulent conditions. Using an appropriate correlation from Chapter 7, determine the minimum velocity needed to sustain the fuel cell at T_c =77°C, and the associated location of the wire. The air and large surroundings are at T_∞ = T_{sur =} 27°C and the fuel cell dissipates E_g = 11W. The fuel cell emissivity is ε = 0.85.

Example 1.5

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A hydrogen-air Proton Exchange Membrane (PEM) fuel cell is illustrated below. It consists of an electrolytic membrane sandwiched between porous cathode and anode materials, form- ing a very thin, three-layer membrane electrode assembly (MEA). At the anode, protons and electrons are generated (2H₂ → 4H+ + 4e); at the cathode, the protons and electrons recom- bine to form water (O₂ + 4e + 4H+ → 2H₂O). The overall reaction is then 2H₂ + O₂ → 2H₂O. The dual role of the electrolytic membrane is to transfer hydrogen ions and serve as a barrier to electron transfer, forcing the electrons to the electrical load that is external to the fuel cell. H₂ E- H₂ LA LA +9 Porous anode H+ H+ H+ L Alr h, T H₂O 0₂ H₂O 0,₂ H₂O Porous cathode -Electrolytic membrane T₂ 9 Tur H₂O + O₂